1. Field of the Invention
This invention relates generally to catalytic dehydrogenation reactions, such as the dehydrogenation of alkyl aromatic compounds to produce vinyl aromatics, for example ethylbenzene into styrene.
2. Description of the Related Art
Alkenyl aromatics, such as vinyl aromatics, are important compounds used in the manufacture of many of today's products. These compounds are commonly produced by catalytic dehydrogenation reactions. For example, styrene can be produced by the catalytic dehydrogenation of ethylbenzene.
Processes for the catalytic dehydrogenation of compounds are well known in the art. An example is the dehydrogenation of an alkyl aromatic compound to yield a corresponding alkenyl aromatic compound. Another example is the dehydrogenation of a mono-olefin to yield a corresponding conjugated di-olefin. Many dehydrogenation catalysts and operating parameters are known, and each can have different advantages and disadvantages. The dehydrogenation catalysts customarily used in such processes are iron oxide based catalysts.
There are a number of factors to consider relative to dehydrogenation catalysts and their particular operation, such as for example between the level of conversion and the useful catalyst life. Catalyst life is an important consideration in dehydrogenation reactions. There are the costs related to the catalyst itself, such as the unit cost of the catalyst, the useful life of the catalyst, the ability to regenerate used catalyst, and the cost of disposing of used catalyst. There are also the costs related to shutting down a dehydrogenation reactor to replace the catalyst and/or to regenerate the catalyst bed, which includes labor, materials, and loss of productivity.
Normal catalyst deactivation can tend to reduce the level of conversion, the level of selectivity, or both, each of which can result in an undesirable loss of process efficiency. There can be various reasons for deactivation of dehydrogenation catalysts. These can include the plugging of catalyst surfaces, such as by coke or tars, which can be referred to as carbonization; the physical breakdown of the catalyst structure; and, the loss of promoters, such as the physical loss of an alkali metal compound from the catalyst. Depending upon the catalyst and the various operating parameters that are used, one or more of these mechanisms may apply.
It is generally preferred to maximize the useful catalyst life, and there are a number of techniques or methods that are known. One technique that is sometimes employed is to raise the reaction temperature. This can be accomplished, for example, by increasing the temperature of the reactant stream or by adding heat to the reactor chamber. Such a reaction temperature increase will generally increase the rate of reaction, which can offset the deactivation of the catalyst, but may also have undesirable results such as harming efficiency or selectivity. There can also be narrow limits to the utility of this temperature-raising technique. There may also be a mechanical temperature limit of the catalyst or the equipment, beyond which further temperature increases can degrade the catalyst's physical structure and/or the equipment's integrity. As this limit is approached, the catalyst would then need to be either replaced or regenerated by conventional ways. Conventional practice generally involves shutting down the reactor and physically removing the catalyst for replacement.
It would be desirable to have a catalyst regeneration method that could be used during steady-state process conditions without process interruption, which would maintain acceptable levels of conversion and selectivity. It is also desirable to have an apparatus to facilitate the addition of the catalyst life extender to the process during steady-state process conditions without process interruption.